Jorge,
The flange portion is different between the two standards..so they will not interchange. As the tool changer retainers would be different. And also the spindle tangs would be different too.
Also a Cat 40 would normally have a Imperial thread for the retaining knob and a BT40 would normally be metric. So probably the retaining knobs from your Cat holder would not fit your BT holders.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)